1. Field of the Invention
The present invention relates to an inverter apparatus which controls an induction motor, and particularly to an inverter apparatus which enables regenerative braking in applications such as an automated storage/retrieval system which handles independently controlled objects for travel and elevation.
2. Description of the Background Art
FIG. 4 is a system configuration diagram where a power regeneration apparatus, available as an option, is connected to a conventional general-purpose inverter apparatus. In this drawing, the numeral 1 indicates an AC power supply of a commercial frequency, 52 represents a general-purpose inverter apparatus which outputs an AC voltage of a required voltage and frequency, 3 designates an induction motor acting as a load of the general-purpose inverter apparatus 52, and 54 denotes a power regeneration apparatus which returns braking energy generated at the braking of the induction motor 3 to the AC power supply 1. The power regeneration apparatus 54 is contained in a casing independent from the general-purpose inverter apparatus 52, and is connected to the DC circuit of the general-purpose inverter apparatus 52 and the AC power supply 1.
In the general-purpose inverter apparatus 52, 5 indicates a diode bridge serving as a converter which converts an AC voltage of a commercial frequency supplied by the AC power supply 1 into a DC voltage, 6 designates a current limiting resistor, 7 represents a smoothing electrolytic capacitor, and 8 denotes switch contacts which are open at the initial charging time of the smoothing electrolytic capacitor 7 to charge the smoothing electrolytic capacitor 7 through the resistor 6 and are closed after completion of the charging to short the resistor 6. An inverse conversion circuit 9, which is composed of six arms each being a parallel connection of a transistor as a switching element and a current-returning diode, reconverts the DC input into an AC voltage of a predetermined voltage and frequency and supplies the induction motor 3 with that AC voltage. Further, 15 denotes a transistor control circuit and 40 represents an arithmetic circuit. The inverse conversion circuit 9 is controlled by the transistor control circuit 15 in accordance with the frequency and voltage calculated by the arithmetic circuit 40. The inverter apparatus 52 includes output terminals 19 for output of an abnormality signal at an occurrence of abnormality, and input terminals 20 for input of a reset signal.
In the power regeneration apparatus 54, 60 designates current limiting resistors, 11 denotes AC reactors, and 12 represents a transistor bridge constituted by six sets of arms where transistors 12a acting as switching devices and diodes 12b for returning current are connected in parallel. A phase detecting circuit 63 is used to detect the phase of the AC power supply 1, a regenerative mode judging circuit 14 is used to judge whether the general-purpose inverter apparatus 52 is in a regenerative mode or not, and a transistor control circuit 70 is used to control the transistor bridge 12 in accordance with the output of the regenerative mode judging circuit 14, in response to signals from an arithmetic circuit 80. To operate the transistor bridge 12 as a converter, the transistor control circuit 70 switches the transistors 12a on/off in synchronization with the AC power supply 1, whereby regenerative operation is performed at the phase where the power factor is approximately 1.
The power regeneration apparatus 54 also has a current detector 16 connected to a DC bus, and an overcurrent judging circuit 17 which judges an overcurrent according to the output from the current detector 16 and whose output is connected to the transistor control circuit 15 to stop the control output if an overcurrent occurs. There also is an AC power presence/absence detecting circuit 18 which detects the presence/absence of the AC voltage 1 in an isolated manner. Finally, the power regeneration apparatus has output terminals 19' for output of an abnormality signal at an occurrence of abnormality in the power regeneration apparatus 54, and input terminals 20' for input of a reset signal for the power regeneration apparatus 54.
FIG. 5 is a system configuration diagram where there are two sets of conventional general-purpose inverter apparatuses and power regenerative apparatuses. In this drawing, 3A and 3B indicates induction motors, 52A and 52B represent general-purpose inverter apparatuses, and 54A and 54B designate power regeneration apparatuses.
Next, the operation of the general-purpose inverter apparatus 52 will be described with reference to FIG. 4. The AC voltage supplied from the AC power supply 1 is converted into the DC voltage by the diode bridge 5, which, at first, charges the electrolytic capacitor 7 through the limiting resistor 6. After the charging is completed, the contacts 8 are closed to allow a current to bypass the limiting resistor 6. The DC voltage as smoothed out by the electrolytic capacitor 7 is reconverted to an AC voltage of a prescribed voltage and frequency by the inverse conversion circuit 9, which is supplied to the induction motor 3 serving as a load.
The AC voltage is also supplied from the AC power supply 1 to the transistor bridge 12 through the power regeneration apparatus 54, i.e., through the resistors 60 and the AC reactors 11. That AC voltage is converted to a DC voltage by the current-returning diodes 12b of the transistor bridge 12, and charges the capacitor 7 of the general-purpose inverter apparatus 52. That is, while the induction motor 3 is operated by the general-purpose inverter apparatus 52, the transistor bridge 12 of the power regeneration apparatus 54 serves, together with the diode bridge 5, as a forward converter.
On the other hand, when the induction motor 3 is braked by returning the energy therefrom, the inverse conversion circuit 9 operates as a forward converter and the regenerative power charges the electrolytic capacitor 7. The rise of the terminal voltage of the electrolytic capacitor 7 is detected by the regenerative mode judging circuit 14 and the transistor bridge 12 is operated as an inverse converter to return the power to the AC power supply 1. In this case, the AC reactors 11 are employed to limit a current change ratio when the transistor bridge 12 is phase-controlled, i.e. produces an effect on the prevention of an overcurrent due to a sudden change or the like in the AC power supply 1. The resistors 60 are used to limit the peak value of the current.
During regeneration, the current is detected by the current detector 16. If a protective function, such as overcurrent protection, is activated in the power regeneration apparatus 54 which provides an overcurrent protection, an abnormality signal is output from the output terminals 19'. For resetting, a reset signal is supplied to the input terminals 20'.
FIG. 6 is an arrangement diagram of the regenerative mode judging circuit 14 in the conventional power regeneration apparatus 54. Referring to this drawing, a DC voltage detection circuit 21 is used to detect the output voltage of the DC circuit of the general-purpose inverter apparatus 52 in an insulated manner, and a phase/voltage detection circuit 22 is used to detect the full-wave rectification output of the AC power supply voltage in an isolated manner. An average value filter 29 serves to average the output of the phase/voltage detection circuit 22. An isolation 61 is used in each of detection circuits 21 and 22. Addition circuit 23 takes a difference between the output of the DC voltage detection circuit 21 and that of the average value filter 29, and the different signal is provided to comparison circuit 26 which compares the output of the addition circuit 23 with a given reference value. Amplifiers 27 and 28 provide gains which are used to match the output levels of the DC voltage detection circuit 21 and the phase/voltage detection circuit 22.
The operation of the regenerative mode judging circuit 14 will now be described. The output V.sub.DC of the DC voltage detection circuit 21, which detects the output voltage of the DC circuit of the general-purpose inverter apparatus 52 in an isolated manner, is matched with the output V.sub.AC of the phase/voltage detection circuit 22, which detects the full-wave rectification output of the AC power supply voltage. If the following expression is established by the comparison circuit 26: ##EQU1## it is judged that the general-purpose inverter apparatus 52 is in the regenerative mode.
It is to be noted that the full-wave rectification output of the AC power supply voltage, which reflects the variations (depression, distortion, instantaneous power failure, instantaneous drop) of the power supply voltage in real time, is filtered by the average value filter 29 including a filter to prevent misoperation, and the gains are adjusted by the gain circuits 27, 28 to adjust the offsets of the DC voltage detection circuit 21 and the phase/voltage detection circuit 22.
FIG. 8 is an arrangement diagram of the power presence/absence detection circuit 18 which detects the presence/absence of the AC voltage in an isolated manner in the conventional power regeneration apparatus 54. In this drawing, 31 indicates a voltage detecting resistor, 32 denotes an isolation circuit, 33 designates resistors for handling the voltage of a control system, and 34 indicates a filtering capacitor.
The operation of the power presence/absence detection circuit 18 will now be described. The AC voltage is applied across the detecting resistor 31 and the AC current flows in the primary circuit of the isolating means 32. As a result, the secondary circuit conducts, whereby the presence/absence of the power supply voltage is converted into the Low/High of the control voltage signal via the resistors 33, which handle the voltage of the control system, and the filtering capacitor 34.
To use the conventional general-purpose inverter in applications where travel and elevation operations are handled as one set, e.g. an automated storage/retrieval system, two sets of the inverter apparatuses 52 and the power regeneration apparatuses 54 are required. Also, since these two sets operate independently of each other, a total of four sets of the apparatuses are required.
However, in the application of the automated storage/retrieval system where travel and elevation are performed as a set of operations, both of the traveling inverter apparatus and elevating inverter apparatus are not always in a regenerative mode or are not always in a driving mode at the same time. On an entire system basis, therefore, the diode bridges 5 of the inverter apparatuses and the transistor bridges 12 of the power regeneration apparatuses are not utilized with high efficiency, respectively.
Also, since the conventional power regeneration apparatus 54 requires three resistors 60 for phases R, S and T to limit the peak value of a regenerative current and these resistors generate heat, the main circuit wiring is complicated.
Also, in the arrangement of the inverter apparatuses 52 and the power regeneration apparatuses 54 as described above, when the inverter apparatus 52A and the power regeneration apparatus 54A on one side are in the driving mode, with the voltage of the capacitor 7 low, and the inverter apparatus 52B and the power regeneration apparatus 54B on the other side are in the regenerative mode as shown in FIG. 5, the DC bus current of the inverter apparatus 52A in the driving mode becomes lower than that of the inverter apparatus 52B in the regenerative mode. Hence, the capacitor 7 of the inverter apparatus 52A in the driving mode is charged by the steep regenerative current to the AC power supply 1 in path A indicated by the arrow, whereby the overcurrent judging circuit 17 of the power regeneration apparatus 54B in the regenerative mode is activated.
When the current changes abruptly or the voltage of the capacitor 7 falls and the power is restored, an overcurrent occurs because a rush current flows in path B to charge the capacitor 7.
Since the conventional power regeneration apparatus 54 is applied as an option for the inverter apparatus 52, the abnormality output terminals 19 of the power regeneration apparatus 54 cannot also serve as the output terminals of the inverter apparatus 52. Therefore, a total of four abnormality output terminals must be processed for the purpose of control in the system. For the same reason, the reset input terminals 20 of the power regeneration apparatus 54 cannot also serve as the input terminals 20 of the inverter apparatus 52. Therefore, a total of four reset input terminals must be processed for the purpose of control in the system.
In the regeneration judging circuit 14 of the conventional power regeneration apparatus 54, the output voltage of the DC circuit of the inverter apparatus 52 and the full-wave rectification output of the AC power supply voltage are provided in an isolated manner by the isolation circuit 61 and compared on a hardware basis. Therefore, if the power is shut off in a stop state after power-on, the voltage of the capacitor 7 is constant because the inverter apparatus 52 is under no load, but the output of the average value filter 29 including the filter reduces toward zero sharply as shown in FIG. 7. Hence, when the output has fallen below the voltage of the capacitor 7 by more than the given value in the comparing means 26, the regenerative mode judging means 14 misjudges it as the regenerative mode.
Also, the configuration in a hardware nature makes the circuit complicated.
In the conventional power regeneration apparatus 54 having the power presence/absence detection circuit 18 which detects the presence/absence of the AC power supply voltage in an isolated manner, if an open phase or cable disconnection occurs in the power supply, the regenerative current flows into the detection circuit itself (see FIG. 9), whereby misdetection is made and an open phase or cable disconnection cannot be detected properly.